Might be useful to do basic data checks and throw some neat ArgumentError

Because if I for some reason pass an empty array to `histogram` the error is cryptic:
```none
gems/2.4.0/gems/unicode_plot-0.0.5/lib/unicode_plot/histogram.rb:12:in `histogram': undefined method `-' for nil:NilClass (NoMethodError)
	from main.rb:85:in `block in <main>'
	from main.rb:84:in `zip'
	from main.rb:84:in `<main>'
```